The Spirogram
A spirogram displays the volume of gas exhaled as a function of time (see
Fig. 22.3A ) and measures: (1) forced vital capacity (FVC), (2) forced expiratory volume in 1 second (FEV1), (3) the ratio of FEV1 to FVC (FEV1/ FVC), and (4) the average midmaximal expiratory flow (FEF25-75).
The total volume of air that is exhaled during a maximal forced exhalation from TLC to RV is called the FVC. The volume of air that is exhaled in the first second during the maneuver is called the FEV1. In normal individuals, 70% to 85% (depending on age) of the FVC can be exhaled in the first second. Thus the normal FEV1/FVC ratio is greater than 70% in healthy adults. A ratio less than 70% suggests •Fig. 22.3 Theclinicalspirogram(A)andflow-volumeloop(B).Theindividualtakesamaximalinspirationandthenexhalesasrapidly,asforcibly,andasmaximallyaspossible.Thevolumeexhaledisplottedasafunctionoftime.Inthespirogramthatisreportedinclinicalsettings,exhaledvolumeincreasesfromthebottomofthetracetothetop(A).Thisisincontrasttothephysiologist’sviewofthesamemaneuver(see
Fig.21.3 ),inwhichtheexhaledvolumeincreasesfromthetoptothebottomofthetrace.Intheflow-volumeloop(B),exhaledvolumeisplottedasafunctionoftheinstantaneousflowrate,whichismeasuredusingapneumotachometer.Themaximalexpiratoryflowrateachievedduringthemaneuveriscalledthepeak expiratory flow rate. NotethelocationsofTLCandRVonbothtracings.
difficulty exhaling because of obstruction and is a hallmark of obstructive pulmonary disease. One expiratory flow rate—the average flow rate over the middle section of the VC—can be calculated from the spirogram. This expiratory flow rate has several names, including MMEF (midmaximal expiratory flow) and FEF25-75 (forced expiratory flow from 25%–75% of VC). Although it can be calculated from the spirogram, today’s spirometers automatically calculate FEF25-75.
Another way of measuring lung function clinically is the flow-volume curve or loop. A flow-volume curve or loop is created by displaying the instantaneous flow rate during a forced maneuver as a function of the volume of gas. This instantaneous flow rate can be displayed both during exhalation (expiratory flow-volume curve) and during inspiration (inspiratory flow-volume curve) (see
Fig. 22.3B ). Expiratory flow rates are displayed above the horizontal line, and inspiratory flow rates are displayed below the horizontal line. The flow-volume loop measures: (1) the FVC, (2) the greatest flow rate achieved during the expiratory maneuver, called the peak expiratory flow rate (PEFR), and (3) multiple expiratory flow rates at various lung volumes. When the expiratory flow-volume curve is divided into quarters, the instantaneous flow rate at which 50% of the VC remains to be exhaled is called the FEF50 (also known as the V̇ max50), the instantaneous flow rate at which 75% of the VC has been exhaled is called the FEF75 (V̇ max75), and the instantaneous flow rate at which 25% of the VC has been exhaled is called the FEF25 (V̇ max25).
•Fig. 22.4 Isovolumecurves.Threesuperimposedexpiratoryflowmaneuversaremadewithincreasingeffort.Notethatpeakinspiratoryandexpiratoryflowratesaredependentoneffort,whereasexpiratoryflowrateslaterinexpirationareindependentofeffort.